This invention relates to a scanning electron microscope (hereinafter referred to as SEM) to obtain images of an observation object, by irradiating a focused electron beam to a semiconductor device in course of manufacture at a front-end semiconductor device process, and detecting electrons released from the irradiating location, and in particular, relates to a SEM type semiconductor wafer inspection apparatus required to obtain high-magnification images, a review SEM to observe defects detected in a semiconductor wafer, in more detail, and further a method for reviewing, in high-magnification, defects having voltage contrast at an inspection apparatus detecting the defects and an apparatus thereof.
With miniaturization of semiconductor devices, it has become increasingly difficult to control a front-end semiconductor device manufacturing process, and it has been important to inspect and review after a circuit is formed. As known examples concerning general inspection and review, an example disclosed in JP-A-10-135288 is well-known, wherein such a sequence is disclosed: inputting, by a review unit, defects coordinates detected by an inspection apparatus using mainly an optical microscope; obtaining low-magnification images after transferring the field of view of a SEM type review apparatus into the coordinates; and after localizing a defect location from the image obtained, obtaining high-magnification images at the defect location. However, by a review, which is also shown in this known example, conducted after inspection by using an optical microscope, with multi-stratification of a semiconductor device pattern, and, further, due to the increase in aspect ratio by finer patterning trend, it has become difficult to observe defects generated at the bottom having trench configuration between patterns, by an optical microscope.
A problem of high aspect ratio is more serious concerning defects generated at the bottom of a hole, and it is almost impossible to detect by an optical inspection apparatus. Therefore, though an inspection based on images obtained by a SEM has prevailed to detect these defects, imaging by a SEM has a problem that time required for imaging by a SEM is generally longer than that by an optical microscope; this disables to inspect with high throughput. To solve this problem, recently a method has been conducted, wherein a test pattern different from an actual circuit of a semiconductor device is formed and only this test pattern is inspected.
As a well-known example of this method, for example, as disclosed in US 2004/0207414, an inspection method is known, wherein, a test pattern, in which voltage contrast (potential contrast) is easily changed, is formed when electrical defects are generated in a semiconductor device, and after detecting first a pattern generating voltage contrast, only the pattern generating voltage contrast is inspected; this enables to inspect relatively at high speed even by a SEM type inspection apparatus.
Further, as a method for distinguishing defects generating voltage contrast from shape defects, as disclosed in U.S. Pat. No. 6,642,726, there is a method, wherein, in the case where a defect size is large and the ratio of a short side to a long side of a rectangular area in an area detected, is large, the defects are classified as ones generating a voltage contrast phenomenon, and it is judged that the defect location is a position where brightness changes in the case of open defects, and a defect area extends across patterns in the case of short defects.
Further, as a method for finding a defect location, though there is no description on voltage contrast change, for example, as described in JP-A-2003-098114, a method is disclosed, wherein, for an image of defects, parts where the same pattern is imaged at different positions, are searched by every local region; and a reference image is composed based on the pattern searched; and a defect location is detected by comparing the defect image with a comparison image composed; this enables to calculate a defect location without imaging a reference image.
Among conventional methods described above, first in the review method disclosed in JP-A-10-135288, it had a problem of difficulty in obtaining a high-magnification image of defects. Generally, with fatal defects generated on a test pattern, voltage contrast change is caused due to electrical characteristic change generated by the defects. In the well-known example, because a high-magnification image is obtained by comparing a defect image with a reference image, and then the difference is detected as defects, the central part of a voltage contrast change area, which is imaged relatively large, is imaged as a defect location. In reviewing defects, however, because what is required is, not confirming that voltage contrast change is generated, but reviewing defects themselves causing the voltage contrast, generally, this method cannot satisfy user's requirement. In addition, in the case where a high-magnification image is obtained only based on defect coordinates output from an inspection apparatus, because stages for moving a wafer are basically different between those in an inspection apparatus and in a review apparatus; this leads to insufficient correspondence between coordinates, and it becomes difficult to obtain a defect image, because defects are not located within a small field of view, which is indispensable in the case of trying to inspect defects at high-magnification.
Further, U.S. Pat. No. 6,642,726 describes, as a method for finding defects on a test pattern in a SEM type inspection apparatus, methods for finding a defect location from defects with voltage contrast by a SEM, including a method for distinguishing voltage contrast defects from a physical defects; and a method for identifying a defect location of defects causing voltage contrast, however, these methods have a problem of inability to stably detect short defects. A SEM type review apparatus is required to review defects with voltage contrast, however, an enlarged field of view of a review apparatus is required to observe defects detected by a SEM type inspection apparatus, by a review apparatus, regardless of alignment error between a SEM type inspection apparatus and a review apparatus. Meanwhile, in the well-known example, it is described that defects which cause short defects are extracted as defects which connect a pattern having voltage contrast difference between a defect image and comparison image thereof, and a pattern adjacent to the pattern, however, in the case of imaging at a wide field of view, a distance between patterns is imaged as quite short. In the case where voltage contrast is generated, it is common that defects are imaged as bright as a pattern and it is difficult to identify a location, in particular, in the case of microscopic defects present between patterns.
Further, as the second problem, in the case where difference of voltage contrast is generated, and in the case where a reference image is composed by using the a method disclosed in JP-A-2003-098114, because difference by voltage contrast is imaged comparatively large, a problem arises of a phenomenon that brightness change remains due to voltage contrast generated by a defect, occurs in a reference image; this makes it impossible to detect the whole of voltage contrast abnormal parts. In particular, in voltage contrast generated from open defects, this becomes a problem to find a defect location. In open defects, as described in U.S. Pat. No. 6,642,726, it is important to find a starting point where voltage contrast difference starts, however, when a reference image is poorly composed, the difference area which is normally one area, is detected as separated ones; this disables to stably find a starting point of voltage contrast.